A hologram is a three-dimensional film record of a physical system which, when replayed, produces a true three-dimensional image of the system. Holography differs from stereoscopic photography in that the holographic image exhibits full parallax by affording an observer a full range of viewpoints of the image from every angle, both horizontal and vertical, and full perspective, i.e. it affords the viewer a full range of perspectives of the image from every distance from near to far. A holographic representation of an image thus provides significant advantages over a stereoscopic representation of the same image. This is particularly true in medical diagnosis, where the examination and understanding of volumetric data is critical to proper medical treatment.
While the examination of data which fills a three-dimensional space occurs in all branches of art, science, and engineering, perhaps the most familiar examples involve medical imaging where, for example, Computerized Axial Tomography (CT or CAT), Magnetic Resonance (MR), and other scanning modalities are used to obtain a plurality of cross-sectional images of a human body part. Radiologists, physicians, and patients observe these two-dimensional data "slices" to discern what the two-dimensional data implies about the three-dimensional organs and tissue represented by the data. The integration of a large number of two-dimensional data slices places great strain on the human visual system, even for relatively simple volumetric images. As the organ or tissue under investigation becomes more complex, the ability to properly integrate large amounts of two-dimensional data to produce meaningful and understandable three-dimensional mental images may become overwhelming.
Other systems attempt to replicate a three-dimensional representation of an image by manipulating the "depth cues" associated with visual perception of distances. The depth cues associated with the human visual system may be classified as either physical cues, associated with physiological phenomena, or psychological cues, which are derived by mental processes and predicated upon a person's previous observations of objects and how an object's appearance changes with viewpoint.
The principal physical cues involved in human visual perception include:
(1) accommodation (the muscle driven change in focal length of the eye to adapt it to focus on nearer or more distant objects); (2) convergence (the inward swiveling of the eyes so that they are both directed at the same point); (3) motion parallax (the phenomenon whereby objects closer to the viewer move faster across the visual field than more distant objects when the observer's eyes move relative to such objects); and (4) stereo-disparity (the apparent difference in relative position of an object as seen by each eye as a result of the separation of the two eyes). The principal psychological cues include: (1) changes in shading, shadowing, texture, and color of an object as it moves; relative to the observer; (2) obscuration of distant objects blocked by closer objects lying in the same line of sight; (3) linear perspective (a phenomenon whereby parallel lines appear to grow closer together as they recede into the distance); and (4) knowledge and understanding which is either remembered or deduced from previous observations of the same or similar objects.
The various psychological cues may be effectively manipulated to create the illusion of depth. Thus, the brain can be tricked into perceiving depth which does not actually exist. However, the physical depth cues are not subject to such manipulation; the physical depth cues, while generally limited to near-range observation, accurately convey information relating to an image. For example, the physical depth cues are used to perceive depth when looking at objects in a small room. The psychological depth cues however, must be employed to perceive depth when viewing a photograph or painting (i.e. a stereoscopic depiction) of the same room. While the relative positions of the objects in the photograph may perhaps be unambiguously perceived through the psychological depth cues, the physical depth cues nonetheless continue to report that the photograph or painting is merely a two-dimensional representation of a three-dimensional space.
Stereo systems depend on image pairs each produced at slightly different perspectives. The differences in the images are interpreted by the visual system (the psychological cues) as being due to relative size, shape, and position of the objects and thus create the illusion of depth. A hologram, on the other hand, does not require the psychological cues to overrule the physical depth cues in order to create the illusion of a three-dimensional image; rather, a hologram produces an actual three-dimensional image.
Conventional holographic theory and practice teach that a hologram is a true three-dimensional record of the interaction of two beams of coherent, mutually correlated light, in the form of a microscopic pattern of interference fringes. More particularly, a reference beam of light is directed at the film substrate at a predetermined angle with respect to the film. An object beam, which is either reflected off of or shines through the object to be recorded, is normally (orthogonally) incident to the film. The reference object beams interact within the volume of space occupied by the film and, as a result of the coherent nature of the beams, produce a standing (static) wave pattern within the film. The standing interference pattern exposes the photographic emulsion within the film, resulting in a pattern of alternating light and dark lines known as interference fringes. The fringe pattern, being a product of the standing wave front produced by the interference between the reference and object beams, literally encodes the amplitude and phase information of the standing wave front. Thus, when the hologram is properly re-illuminated, the amplitude and phase information encoded in the fringe pattern is replayed in free space, producing a true three-dimensional image of the object.
Conventional holographic theory further suggests that a sharp, well defined fringe pattern produces a sharp, bright hologram, and that an overly strong object beam will act like one or more secondary reference beams causing multiple fringe patterns to form (intermodulation) and diluting the strength of the primary fringe pattern. Accordingly, holographers typically employ a reference beam having an amplitude at the film surface approximately five to eight times that of the object beam to promote the formation of a single high contrast pattern within the interference fringe pattern and to reduce spurious noise resulting from bright spots associated with the object. Moreover, since known holographic techniques generally surround the recording of a single hologram or, alternatively, up to two or three holograms, onto a single film substrate, the stated objective is to produce the strongest fringe pattern possible to ensure the brightest holographic display. Accordingly, holographers typically attempt to expose the maximum number of photosensitive grains within the film emulsion while the object is being exposed. Since every point within the holographic film comprises a fringe pattern which embodies information about every point on the object, fringe patterns exist throughout the entire surface of the holographic film, regardless of the configuration of the object or image which is the subject of the hologram. Consequently, the creation of strong, high contrast fringe patterns necessarily result in rapid consumption of the finite quantity of photosensitive elements within the emulsion, thereby limiting the number of high contrast holograms which can be produced on a single film substrate to two or three. Some holographers have suggested that as many as 10 to 12 different holographic images theoretically may be recorded on a single film substrate; however, heretofore, superimposing more than a small finite number of holograms has not been recognized, and in fact, utilizing conventional hologram theory has not been possible.
In prior art holograms employing a small number of superimposed holographic images on a single film substrate, the existence of a relatively small percentage of spurious exposed photosensitive elements (fog) does not appreciably degrade the quality of the resulting hologram. In contrast holograms made in accordance with the subject invention, discussed below, typically employ up to 100 or more superimposed holograms on a single film substrate; hence, the presence of a small amount of fog on each hologram would have a serious cumulative effect on the quality of the final product.
A method and apparatus for producing holograms is therefore needed which permits several hundred or more different holograms to be recorded on a single film substrate, thereby facilitating the true, three-dimensional holographic reproduction of human body parts and other physical systems which are currently viewed in the form of two-dimensional data slices.